/Engine/Source/Runtime/Engine/Classes/Engine/GameViewportClient.h
Adds a widget to the Slate viewport's overlay (i.e for in game UI or tools) at the specified Z-order
virtual void AddViewportWidgetContent ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< class [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > ViewportContent, const int32 ZOrder )